A pacemaker is an implantable device that delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia—a condition in which a patient cannot maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate, which was typically set at a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, although in more advanced devices, the rate could be set remotely by a medical practitioner after implantation.
Early advances in pacemaker technology included the ability to sense intrinsic cardiac activity, i.e., the intracardiac electrogram, or “IEGM” signal. This led to the development of “demand pacemakers,” so named because they deliver stimulation pulses only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous cardiac contraction which occurs within a predetermined time period (commonly referred to as the “escape interval”) following a preceding contraction, whether spontaneous or evoked by the implantable device. When a naturally occurring contraction is detected within the escape interval, a demand pacemaker does not deliver a pacing pulse.
Pacemakers such as those described above proved to be extremely beneficial in that they successfully reduced or eliminated seriously debilitating and potentially lethal effects of bradycardia in many patients. However, since pacemakers are implantable devices, an invasive surgical procedure is required, and many patients who receive pacemakers must undergo several surgical procedures, because pacemakers have a limited life span, due to limited battery life, and therefore require periodic replacement. Of course, it is desirable to minimize the number of surgical procedures that must be performed on a patient to improve safety and reduce costs, not to mention patient inconvenience and discomfort.
The life span of most pacemakers is directly related to the rate at which their batteries drain. Thus, a substantial effort has been directed toward minimizing the amount of energy used by pacemakers, while ensuring that the devices continue to deliver effective therapy. Demand pacemakers effectively reduce battery drain by delivering pacing pulses only when required. However, each pacing pulse delivered by a demand pacemaker may have a significantly higher energy content than that required for inducing a cardiac contraction. Thus, even after the development of demand pacemakers, there remained an opportunity for further improvements in the area of pacemaker energy utilization.
The minimum amount of electrical stimulation that effectively evokes a cardiac contraction is commonly referred to as a patient's “capture threshold.” Unfortunately, capture threshold varies significantly among patients; therefore, the amount of electrical stimulation provided by a pacemaker cannot be permanently set by the manufacturer. Rather, stimulus parameters must be individually set for each patient immediately after implantation and during subsequent office visits.
Determining a particular patient's capture threshold is a relatively simple procedure when performed during an office visit. Essentially, the medical practitioner can remotely adjust the amount of electrical stimulation downward from a maximum value that is known to elicit a contraction for all patients. Once the amount of electrical stimulation falls below the patient's capture threshold, an ensuing heartbeat is not detected, and the medical practitioner increases the amount of electrical stimulation beyond the last successful level.
Typically, a substantial safety margin is added to the measured capture threshold to ensure that the pacemaker continues to evoke contractions over an extended period of time. The safety margin is necessary because a patient's capture threshold varies over time—sometimes dramatically during the first few months following implantation. However, by adding such a large safety margin, it is almost assured that the pacemaker will be wasting significant amounts of energy during its life span.
In an effort to reduce the amount of energy wasted, pacemakers have been developed that periodically (or even continually) evaluate the patient's capture threshold during normal operation. These devices are also capable of automatically adjusting the amount of electrical stimulation in response to changes to the capture threshold. These features, which in combination are referred to herein as “automatic capture detection”, significantly reduce unnecessary battery drain, because higher energy pacing pulses are delivered only when needed by the patient. Although most of these devices continue to add a safety margin to the measured capture threshold, the safety margin can be greatly reduced, especially when the capture threshold is measured frequently.
Pacemakers that perform automatic capture detection commonly monitor a patient's IEGM signal to determine what pulsing energy level is necessary to evoke a responsive cardiac contraction (“evoked response”). In particular, the pacemaker samples the portion of the patient's IEGM signal corresponding to the evoked response, if any, immediately after a pacing pulse is delivered. The shape of the waveform indicates whether the pacing pulse successfully captured the heart. However, known automatic capture detection methods have several drawbacks, particularly relating to signal processing, which have proven difficult to overcome. For example, it is extremely difficult to accurately sense the evoked response immediately after a pacing pulse is delivered, due to the presence of residual electrical effects in the immediate vicinity of the pacing electrodes. These residual effects (commonly known as “polarization”) interfere with the pacemaker's ability to sense the evoked response. Indeed, most pacemakers enter a refractory period immediately after a pacing pulse is delivered, during which time the sensing circuitry is deactivated, for the specific purpose of avoiding undesirable sensing of polarization.
One automatic capture system and method is described in U.S. Pat. No. 5,350,410 ('410 patent) entitled “Autocapture System for Implantable Pulse Generator” (Kleks et al.), which is hereby incorporated by reference as if set forth fully herein. The '410 patent discloses a capture verification test for determining the polarization template and which sensitivity settings yield capture. In addition, the '410 patent also discloses an auto-threshold routine for automatically setting the output energy of the normal stimulation pulse. Thus, typical capture tracking systems allow reduced power usage for implantable pacemakers and ICDs, without compromising patient safety.
Traditional capture tracking systems, however, fail to identify fusion beats. A fusion beat is a cardiac depolarization (either atrial or ventricular) that results from two foci. In pacing, a fusion beat typically refers to the electrocardiogram (“ECG” or “EKG”) waveform which results when an intrinsic depolarization and a pulse generator output pulse occur simultaneously (or nearly simultaneously), and both contribute to the electrical activation of the heart chamber. A pseudofusion beat is a spontaneous cardiac depolarization occurring simultaneously (or nearly simultaneously) with a pulse generator output pulse, where the output pulse does not contribute to the cardiac depolarization but nonetheless distorts the morphology of the waveform on the ECG. A pseudopseudofusion beat is an electrocardiographic superimposition of an atrial stimulus on a native QRS complex in a ventricle. Hereinafter, fusion beats, pseudofusion beats and pseudopseudofusion beats are referred to collectively as “fusion beats.”
Because conventional capture tracking systems do not identify fusion beats, these occurrences are typically treated as loss of capture. If a capture tracking system cannot positively identify a sensed signal as capture, loss of capture is assumed in order to guarantee patient safety. But this approach to pacing causes the capture tracking system to routinely issue unnecessary backup pulses, initiate unnecessary threshold searches, and possibly find incorrect capture threshold pulse energies. These problems are particularly pronounced in patients who have atrial fibrillation.
Some have suggested a modification to traditional capture tracking systems and methods, whereby fusion beats may be avoided. In this modification, the escape interval is increased slightly for the next cardiac cycle after a loss of capture event. If a spontaneous cardiac depolarization is then detected before the elongated escape interval, this indicates that the prior loss of capture event was likely due to fusion activity. Thus, the escape interval is maintained at the longer time period until a spontaneous depolarization does not occur in time, at which point the escape interval is reduced to its programmed length.
This modification of a capture tracking system helps to avoid treatment of fusion activity as loss of capture, particularly in devices using dual chamber sensing. It does not, however, eliminate the existing problems in the art. Even with this modification of a capture tracking system, unnecessary backup pulses may still be issued, unnecessary threshold searches may be initiated, and incorrect capture threshold pulse energies may be found. Moreover, because all loss of capture events are assumed to be potential fusion beats, the modification is employed unnecessarily at times.